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What are Bt- maize, Bt-potato, Bt-cotton?

Bt is the abbreviation for Bacillus thuringiensis , a gram-positive bacterium that is common in soil. These bacteria show toxic effects to some classes of insects. In contrast to that Bt-toxin shows no toxic effects on mammals and birds. The toxic effect of these bacteria is used to create plants that possess resistance against a broad range of insects. The genes that produce the toxin are introduced into the crop through vectors, organisms that are able to introduce their genes into plants. In that way the plant later shows the same ability for toxin-production as Bacillus thuringiensis . In that way the plants produce their own insecticide. However, not all harmful insects are affected by that toxin; for example, it has no toxic effect on aphids, but just a reduced amount of chemical insecticides is still needed. Conversely, it has some effects on non-target insects, which is one point of criticism against the application of these transgenic crops. On the other hand, chemical insecticides are also not selective and kill non-target organisms as well. For more detailed description see the background of transgenic crops.